The present invention generally relates to the preparation of semiconductor grade single crystal silicon which is used in the manufacture of electronic components. More particularly, the present invention relates to a process for preparing single crystal silicon, in ingot or wafer form, wherein crucible rotation is utilized to control the average axial temperature gradient in the crystal, G0, as a function of radius (i.e., G0(r)), particularly at or near the central axis. Additionally, crucible rotation modulation is utilized to obtain an axially uniform oxygen content. In one particular embodiment, the present invention is directed to a process for preparing single crystal silicon having an axially symmetric region which is substantially devoid of agglomerated intrinsic point defects using crucible rotation to improve radial uniformity of G0.
Single crystal silicon, which is the starting material in most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by, for example, decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
It is now recognized that a number of defects in single crystal silicon form in the growth chamber as the ingot cools from the temperature of solidification. More specifically, as the ingot cools intrinsic point defects, such as crystal lattice vacancies or silicon self-interstitials, remain soluble in the silicon lattice until some threshold temperature is reached, below which the given concentration of intrinsic point defects becomes critically supersaturated. Upon cooling to below this threshold temperature, a reaction or agglomeration event occurs, resulting in the formation of agglomerated intrinsic point defects.
As has been reported elsewhere (see, e.g., U.S. Pat. Nos. 5,919,302; 6,254,672; 6,287,380; 6,328,795; and, 6,312,516, all of which are incorporated herein by reference), the type and initial concentration of these point defects in the silicon are determined as the ingot cools from the temperature of solidification (i.e., about 1410° C.) to a temperature greater than about 1300° C. (i.e., about 1325° C., 1350° C. or more); that is, the type and initial concentration of these defects are controlled by the ratio v/G0, where v is the growth velocity and G0 is the average axial temperature gradient in the crystal over this temperature range. Specifically, referring now to FIG. 1, for increasing values of v/G0, a transition from decreasingly self-interstitial dominated growth to increasingly vacancy dominated growth occurs near a critical value of v/G0 which, based upon currently available information, appears to be about 2.1×10−5 cm2/sK, where G0 is determined under conditions in which the axial temperature gradient is constant within the temperature range defined above. Accordingly, process conditions, such as growth rate (which affect v), as well as hot zone configurations (which affect G0), can be controlled to determine whether the intrinsic point defects within the single crystal silicon will be predominantly vacancies (where v/G0 is generally greater than the critical value) or self-interstitials (where v/G0 is generally less than the critical value).
Agglomerated defect formation generally occurs in two steps; first, defect “nucleation” occurs, which is the result of the intrinsic point defects being supersaturated at a given temperature. Once this “nucleation threshold” temperature is reached, intrinsic point defects agglomerate. The intrinsic point defects will continue to diffuse through the silicon lattice as long as the temperature of the portion of the ingot in which they are present remains above a second threshold temperature (i.e., a “diffusivity threshold”), below which intrinsic point defects are no longer mobile within commercially practical periods of time. While the ingot remains above this temperature, vacancy or interstitial intrinsic point defects diffuse through the crystal lattice to sites where agglomerated vacancy defects or interstitial defects, respectively, are already present, causing a given agglomerated defect to grow in size. Growth occurs because these agglomerated defect sites essentially act as “sinks,” attracting and collecting intrinsic point defects because of the more favorable energy state of the agglomeration.
Accordingly, the formation and size of agglomerated defects are dependent upon the growth conditions, including v/G0 (which impacts the initial concentration of such point defects), as well as the cooling rate or residence time of the main body of the ingot over the range of temperatures bound by the “nucleation threshold” at the upper end and the “diffusivity threshold” (which impacts the size and density of such defects) at the lower end. As has been previously reported (see, e.g., the U.S. patents referenced above), control of the cooling rate or residence time enables the formation of agglomerated intrinsic point defects to be suppressed over much larger ranges of values for v/G0; that is, controlled cooling allows for a much larger “window” of acceptable v/G0 values to be employed while still enabling the growth of substantially defect-free silicon.
However, with respect to the type and initial concentration of intrinsic point defects, it is recognized that at the time of solidification the pull rate, and thus essentially v, is substantially constant as a function of radius at that particular axial position; that is, v is essentially constant radially at each axial position in the ingot. It is further recognized that typically the concentration, and possibly the type, of intrinsic point defects is not constant along the radius due to the heat-loss at the outer or lateral surface of the ingot, which therefore causes G0 to increase along the radius moving from the center of the ingot outward. In general, this is due to the limitations on hot zone design (i.e., the inability to sufficiently limit heat loss at the crystal surface). This inability to control G0(r) becomes more pronounced as the actual value of G0 increases. As a result, increasing the throughput of substantially defect-free silicon becomes more difficult (an increase in v typically requiring an increase in G0 in order to maintain the desired type and initial concentration of point defects).